Communication terminal with bandwidth widening expansion and echo compensation

ABSTRACT

A communication terminal is provided having a signal reception path which has a bandwidth widening device for artificially widening the bandwidth of a received signal in the communication terminal, a digital/analog converter and a loudspeaker, and includes a signal transmission path, which has a microphone, a transmission path low-pass filter and an analog/digital converter, in which case an echo compensation device is provided between one output of the bandwidth widening device and a connecting point of the signal transmission path, beyond the analog/digital converter with respect to the microphone, and further provides a method for artificially widening the bandwidth of a received signal in a communication terminal.

BACKGROUND OF THE INVENTION

It is known from the prior art for a communication terminal having asignal reception path, which has a bandwidth widening device forartificially widening the bandwidth of a received signal in thecommunication terminal, to have a digital/analog converter and aloudspeaker, and for such a communication terminal having a signaltransmission path to have a microphone, a transmission path low-passfilter and an analog/digital converter. However, no such communicationterminal has yet been introduced to the market.

The intended bandwidth widening device is directed toward widening thebandwidth of a narrowband received signal; to be precise, by samplingand evaluation of the received signal via a suitable algorithm. Forexample, the received signal may be in a frequency band between 300 and3,400 Hz, so that the bandwidth can be widened both in the direction oflow frequencies and in the direction of high frequencies.

Frequency analysis of the narrowband received signal leads, for example,to fundamentals being added on the low-frequency side of the narrowbandreceived signal, whose harmonics are contained in the narrowbandreceived signal. In a similar way, the narrowband received signal maycontain fundamentals whose harmonics can be added to the high-frequencyside of the narrowband received signal. Overall, this results in animproved tonal impression of a sound signal which is emitted via theloudspeaker and is based on the received signal whose bandwidth has beenwidened. In particular, this improves the subjectively perceived qualityof the sound signal, which is predominantly a speech signal.

So-called echo compensation devices likewise are known from the priorart, which seek to suppress the sound signals, which are emitted via theloudspeaker, from the signal reception path on the signal transmissionpath. This is necessary in order to take account of the fact that thesound signals which are emitted from the loudspeaker are emitted onceagain via the microphone of the communication terminal and via anantenna of the communication terminal. As such, a call partner of a userof the communication terminal perceives his/her own speech signals as anecho.

The algorithms which are used for echo compensation have thecharacteristic that they cannot compensate for non-linear distortion ofreceived signals. However, such non-linear distortion results from thebandwidth widening of the received signal as described above.

Against this background, the present invention is directed towardproviding a communication terminal having a bandwidth widening device,in which echoes that occur can be effectively compensated for. A furtheraim is to specify a method for artificially widening the bandwidth of areceived signal in a communication terminal, in which measures are takenfor echo compensation.

SUMMARY OF THE INVENTION

With regard to such communication terminal, the present invention offersa communication terminal having a signal reception path, which has abandwidth widening device for artificially widening the bandwidth of areceived signal in the communication terminal, a digital/analogconverter and a loudspeaker, and includes a signal transmission path,which has a microphone, a transmission path low-pass filter and ananalog/digital converter, in which case an echo compensation device isprovided between one output of the bandwidth widening device and aconnecting point of the signal transmission path, beyond theanalog/digital converter with respect to the microphone.

The present invention thus provides for the output signal from thebandwidth widening device to be made available to the echo compensationdevice, such that the latter can use the algorithm associated with it tosubtract the widened received signal from a signal on the signaltransmission path.

It must be noted that the non-linear distortion in the received signalresulting from the bandwidth widening is contained in the input signalfor the echo compensation device as signal components, so that the echocompensation device can be used in the known manner.

In one embodiment of the present invention, the bandwidth wideningdevice operates at a first sampling rate, and the echo compensationdevice operates at a second sampling rate. A sampling rate conversiondevice is provided for conversion of an output signal from the bandwidthwidening device at the first sampling rate to the second sampling rate,and the output of the sampling rate conversion device is connected to aninput of the echo compensation device.

This procedure takes account of the fact that the bandwidth wideningdevice and the echo compensation device may operate using differentsampling rates, although the echo compensation device should retain aninput signal whose sample rate corresponds to that sampling rate whichis used by the echo compensation device. In many cases, the samplingrate of the bandwidth widening device is higher than that of the echocompensation device, so that the sampling rate conversion device has toreduce the sampling rate.

The sampling rate conversion device may interact with a conversionlow-pass filter which has a pass characteristic which is matched to thesecond sampling rate for the echo compensation device. This embodimentrelates to situations in which the first sampling rate is higher thanthe second sampling rate. In this case, it is important to avoid theinput signal for the echo compensation device containing signalcomponents whose frequency cannot be represented on the basis of thesecond sampling rate. The conversion low-pass filter is used for thispurpose.

The first sampling rate for the bandwidth widening may be 16 kHz, andthe second sampling rate for the echo compensation device may be 8 kHz.These values are typical values for a bandwidth widening device and foran echo compensation device, respectively, but they previously have notbeen combined with one another in the prior art.

In order to optimize the quality of the echo compensation, the passcharacteristic of the conversion low-pass filter may be designed to passsignal components whose frequency is at least as high as that of thetransmission path low-pass filter. If not, the conversion low-passfilter would suppress signal components which then could not becompensated for by the echo compensation device.

With regard to the inventive method, this is achieved by a method forartificially widening the bandwidth of a received signal in acommunication terminal having a signal reception path and a signaltransmission path, having the following successive steps:

-   -   a) sampling of the received signal in the signal reception path;    -   b) widening of the bandwidth of the received signal via a        bandwidth widening algorithm on the basis of sample values        obtained in step a), in order to obtain a widened received        signal; and    -   c) compensation for the echo on the widened received signal for        the signal transmission path via an echo compensation algorithm,        with the widened received signal being sampled.

The individual method steps have already been explained above withreference to the description of the communication terminal with abandwidth widening device and an echo compensation device.

It should be noted that the communication terminal with a bandwidthwidening device and an echo compensation device can, in principle,operate with any combination of a first sampling rate and secondsampling rate. In practice, the most frequent situation will be that inwhich the first sampling rate is higher than the second sampling rate;for example, twice the second sampling rate. An important factor for thepresent invention is, however, that the sampling rate of the outputsignal from the bandwidth widening device is matched to the samplingrate of the echo compensation device, in which it is also possible toincrease the sampling rate for the output signal from the bandwidthwidening device; for example, by interpolation.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the Figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a diagram to illustrate a frequency spectrum of a receivedsignal whose bandwidth has been widened for a communication terminalaccording to the present invention.

FIG. 2 shows an overview block diagram of a circuit part of thecommunication terminal, in order to illustrate bandwidth widening andecho compensation.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the magnitude of the amplitude of signal components of awidened received signal as a function of the frequency. A central signalcomponent NB originates from the received signal which has reached acommunication terminal. In the present exemplary embodiment, thefrequency range of the received signal extends from about 300 Hz toabout 3,400 Hz, although the signal amplitude does not fall awaysuddenly but over a specific interval both at the high-frequency end andat the low-frequency end of the received signal.

A bandwidth widening device ABE (see FIG. 2), which is provided in asignal reception path in the communication terminal, ensures that theoriginal bandwidth of the received signal is widened; to be precise,both at the high-frequency end and at the low-frequency end. Signalcomponents HBE which are added at the high-frequency end overlap thecentral signal component NB of the original received signal. In thisway, both signal components which originate from the original receivedsignal and signal components which have been generated by the bandwidthwidening device ABE occur and overlap in a frequency band of around3,400 Hz. Such overlapping should be regarded as desirable since this isused to optimize the acoustic characteristics of a widened receivedsignal. The same also applies to the low-frequency end of the originalreceived signal; that is to say, the central signal component NB. Signalcomponents LBE are added at its low-frequency end, with the signalcomponents LBE and the central signal components NB overlapping in theband around 300 Hz.

The added signal components LBE, HBE take care of non-linear distortionin the stated overlapping areas, whose effects in terms of echoes cannotbe suppressed by simple filtering. The overlapping areas that have beenmentioned are shown by dashed lines for illustration purposes in FIG. 1.

The design of a circuit part of the communication terminal asillustrated in FIG. 2 has a signal reception path and a signaltransmission path. The signal reception path is characterized by abandwidth widening device ABE, to whose input the original 8 kHzreceived signal, which has reached the communication terminal, isapplied. Bandwidth widening is carried out via a suitable algorithm forbandwidth widening using a sampling rate of 16 KHz, which results in theoutput signal from the bandwidth widening device ABE having a frequencyspectrum which is comparable to that shown in FIG. 1.

The output signal from the bandwidth widening device ABE, which is at asampling rate of 16 kHz, is supplied to a digital/analog converter 1,whose output signal is converted via a loudspeaker LS to a sound signal.

The sound signal that is emitted from the loudspeaker and is providedfor a user of the communication terminal is also passed into amicrophone MIC in the communication terminal which, in principle, isused for recording speech signals, which are returned to the user of thecommunication terminal. In this way, the sound signal which is emittedfrom the loudspeaker LS occurs as an echo in the sound signal recordedby the microphone MIC.

The microphone MIC forms a first element of the signal transmissionpath. Its output signal is supplied to a transmission path low-passfilter TP2, whose flank is matched to a sampling rate of a downstreamanalog/digital converter 2. In the present exemplary embodiment, theanalog/digital converter 2 operates at a sampling rate of 8 kHz, so thatthe transmission path low-pass filter TP2 should attenuate signalfrequencies above 4 kHz in order to satisfy the Nyquist condition thatis appropriate here.

The output signal from the analog/digital converter 2 now contains bothsignal components which result from speech signals produced by the userof the communication terminal and signal components in the form ofechoes which originate from the loudspeaker LS.

The components of the illustrated circuit that will be described now areof particular importance for suppression of echoes. The output signalfrom the bandwidth widening device ABE, which is at a first samplingrate of 16 kHz, is supplied to a conversion low-pass filter TP1 in theform of a decimation low-pass filter. The output signal from theconversion low-pass filter TP1 is passed to a sampling rate conversiondevice ARU, which converts the first sampling rate of 16 kHz to a secondsampling rate of 8 kHz. In this case, the conversion low-pass filter TP1ensures adequate attenuation of signal components at a frequency of morethan 4 kHz applied to its input. In principle, the conversion low-passfilter should not attenuate signal components at a frequency which isalso passed by the transmission path low-pass filter TP2. This ensuresthat echoes which are passed through the transmission path low-passfilter TP2 also may be compensated for by an echo compensation deviceAEC, which will now be described.

The echo compensation device AEC uses a suitable algorithm forcompensation for echoes in the output signal from the analog/digitalconverter 2, and operates at a second sampling rate of 8 kHz. The outputsignal from the echo compensation device AEC is applied to a connectingpoint 3 in the signal transmission path, which is also connected to anoutput of the analog/digital converter 2. Echo compensation is carriedout at the connecting point 3; to be precise, with the aid of acompensation signal which is generated by the echo compensation deviceAEC via the echo compensation algorithm. The sampling rate of thecommunication signal corresponds to the sampling rate of the outputsignal from the analog/digital converter 2.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the present invention as set forth in the hereafter appended claims.

1-7. (canceled)
 8. A communication terminal, comprising: a signal reception path which includes a bandwidth widening device for artificially widening a bandwidth of a received signal in the communication terminal, a digital/analog converter and a loudspeaker; and a signal transmission path which includes a microphone, a transmission path low-pass filter and an analog/digital converter; wherein an echo compensation device is provided between one output of the bandwidth widening device and a connecting point of the signal transmission path, beyond the analog/digital converter with respect to the microphone.
 9. A communication terminal as claimed in claim 8, wherein the bandwidth widening device operates at a first sampling rate, the echo compensation device operates at a second sampling rate which is different from the first sampling rate, and a sampling rate conversion device is provided for conversion of an output signal from the bandwidth widening device at the first sampling rate to the second sampling rate, and an output of the sampling rate conversion device is connected to an input of the echo compensation device.
 10. A communication terminal as claimed in claim 9, wherein the sampling rate conversion device interacts with a conversion low-pass filter having a pass characteristic which is matched to the second sampling rate for the echo compensation device, with the first sampling rate being higher than the second sampling rate.
 11. A communication terminal as claimed in claim 9, wherein the first sampling rate for the bandwidth widening device is 16 kHz, and the second sampling rate for the echo compensation device is 8 kHz.
 12. A communication terminal as claimed in claim 10, wherein the pass characteristic of the conversion low-pass filter for passing signal components is at least as high as a frequency of the transmission path low-pass filter.
 13. A method for artificially widening a bandwidth of a received signal in a communication terminal having a signal reception path and a signal transmission path, the method comprising: sampling the received signal in the signal reception path; widening the bandwidth of the received signal via a bandwidth widening algorithm based on sample values obtained in the step of sampling so as to obtain a widened received signal; and compensating for an echo on the widened received signal for the signal transmission path via an echo compensation algorithm, with the widened received signal being sampled.
 14. A method for artificially widening a bandwidth of a received signal in a communication terminal as claimed in claim 13, wherein the step of widening is carried out at a first sampling frequency, the sampling in the step of compensating is carried out at a second sampling frequency, which is different from the first sampling frequency, and a widened received signal obtained in the step of widening is converted to the second sampling frequency before the step of compensating is carried out. 